IComparer does not implement interface member - Error CS0738

Question

I am trying to sort two objects by one of their properties (.Transaction.topLeftX, an integer) using the following code to create a comparer to use in a Sort method:

public class RespComp : IComparer<Kairos.Net.RecognizeImage>
{     
        public Kairos.Net.RecognizeImage Compare(Kairos.Net.RecognizeImage x, Kairos.Net.RecognizeImage y)
        {
            if (x.Transaction.topLeftX.CompareTo(y.Transaction.topLeftX) <= 0) return x;
               else return y;                      
        }
}

However, I get the error message Error CS0738 'RecogniseFacesKairos.RespComp' does not implement interface member 'IComparer.Compare(RecognizeImage, RecognizeImage)'. 'RecogniseFacesKairos.RespComp.Compare(RecognizeImage, RecognizeImage)' cannot implement 'IComparer.Compare(RecognizeImage, RecognizeImage)' because it does not have the matching return type of 'int'.

Does the comparer used in the Sort method need to have return type int?

Answer 1

The IComparer<T> interface is supposed to implement a method that returns an int comparison. -1 for less than, 0 for equal and 1 for greater than.

Look at your code, if you're just comparing the top left, you can probably just do the following:

public int Compare(FooImage x, FooImage y) {
    return x.Transaction.topLeftX.CompareTo(y.Transaction.topLeftX);
}

Answer 2

The desired outcome of sorting objects by one of their parameters was achieved by the following code:

...
Kairos.Net.KairosClient client = new Kairos.Net.KairosClient();
client.ApplicationID = appId;
client.ApplicationKey = appKey;
Kairos.Net.RecognizeResponse resp = client.Recognize(...);

RespComp SortImages = new RespComp();
resp.Images.Sort(SortImages);
...

 public class RespComp : IComparer<Kairos.Net.RecognizeImage>
    {
        public int Compare(Kairos.Net.RecognizeImage x, Kairos.Net.RecognizeImage y)
        {
            return x.Transaction.topLeftX.CompareTo(y.Transaction.topLeftX);
        }
    }